A few Hardware/OEM vendors tend to screw up BIOS settings, thus rendering the snd_hda driver useless, which usually results in a state where the snd_hda driver seems to attach and work, but without any sound.

20090107:
snd_hda(4) driver was updated to version 20081226_0122.
Due to added HDMI audio and logical audio devices support, updated
driver often provides several PCM devices. In some cases it can make
system default audio device no longer correspond to the users's
habbitual audio connectors. In such cases wanted device can be
specified in audio application setup or defined globally via
hw.snd.default_unit sysctl according to sound(4) and snd_hda(4)
man pages.